Ode to Recall: To remember events in order, we rely on the Brain’s ‘Symphony’

Newswise, October 17, 2016 — To remember events in the order they occur, the brain’s neurons function in a coordinated way that is akin to a symphony, a team of New York University scientists has found. Their findings offer new insights into how we recall information and point to factors that may disrupt certain types of memories.

“The findings enhance our understanding of how the brain keeps track of what happened and when it happened relative to other events,” explains Lila Davachi, associate professor in NYU’s Department of Psychology and Center for Neural Science and the study’s senior author.

“We’ve known for some time that neurons increase their activity when we encode memories. What our study shows is there’s a rhythm to how they fire in relation to one another—much like different instruments in a symphony orchestra.”

The study’s first author was Andrew Heusser, a doctoral candidate in NYU’s Department of Psychology. Its collaborators were David Poeppel, a professor in NYU’s Department of Psychology and Center for Neural Science, and Youssef Ezzyat, also a doctoral candidate in NYU’s Department of Psychology at the time of the research and now a postdoctoral fellow at the University of Pennsylvania.

The research, which appears in the journal Nature Neuroscience, sought to determine the validity of a long-standing hypothesis, proposed in 1995 by neuroscientists John Lisman and Marco Idiart, which outlines how the order of memories is encoded.

The “theta-gamma phase coding” model states that when our brains create a memory for a specific event, our neurons oscillate in a coordinated fashion, with cells firing at high (gamma) frequencies. To encode the order of multiple events, cells representing each event fire in a sequence that is coordinated by a lower (theta) frequency brain rhythm.

To test this, the scientists had the study’s participants view a series of six objects (e.g., a butterfly, headphones, etc.), one at a time, on a computer screen.

During the experiment, researchers examined the subjects’ neural activity using magnetoencephalography (MEG), which captures measurements of the tiny magnetic fields generated by the brain.

Later, they asked subjects to recall the order of the objects they viewed.

In their analysis, the researchers examined the neuronal activity of the subjects when they first viewed the objects, then matched it to the results of the recall test.

Their data showed notable differences in the patterns of neural activity when the order of the objects was correctly encoded compared to when it was not.

Specifically, when the order of the objects was correctly encoded, the gamma activity associated with each object was temporally ordered along a slower theta oscillation so that the gamma activity for object 1 preceded that for object 2 and so on.

By contrast, when subjects incorrectly recalled the order in which the objects were presented, gamma activity was just as high—but there was no discernible pattern.

“When particular oscillations are in step with each other, we remember the order,” Davachi observes. “But when they are not, we don’t.”

The research was supported by a grant from the National Institute of Mental Health (RO1–MH074692).

Don’t I Know That Guy? Neuroscientists pinpoint part of the brain that deciphers memory from new experience

Newswise, August 25, 2015 — You see a man at the grocery store. Is that the fellow you went to college with or just a guy who looks like him?

One tiny spot in the brain has the answer.

Johns Hopkins University neuroscientists have identified the part of the hippocampus that creates and processes this type of memory, furthering our understanding of how the mind works, and what’s going wrong when it doesn’t. Their findings are published in the current issue of the journal Neuron.

“You see a familiar face and say to yourself, ‘I think I’ve seen that face.’ But is this someone I met five years ago, maybe with thinner hair or different glasses — or is it someone else entirely,” said James J. Knierim, a professor of neuroscience at the university’s Zanvyl Krieger Mind/Brain Institute who led the research.

 “That’s one of the biggest problems our memory system has to solve.”

Neural activity in the hippocampus allows someone to remember where they parked their car, find their home even if the paint color changes, and recognize an old song when it comes on the radio.

Brain researchers theorized that two parts of the hippocampus (the dentate gyrus and CA3) competed to decide whether a stimulus was completely new or an altered version of something familiar. 

The dentate gyrus was thought to automatically encode each stimulus as new, a process called pattern separation. In contrast, CA3 was thought to minimize any small changes from one experience to the next and classify the stimuli as being the same, a process called pattern completion. 

So, the dentate gyrus would assume that the person with thinner hair and unfamiliar glasses was a complete stranger, while CA3 would ignore the altered details and retrieve the memory of a college buddy.

Prior work by Knierim’s group and others provided evidence in favor of this long-standing theory. 

The new research shows, however, that CA3 is more complicated than previously thought — parts of CA3 come to different decisions, and they pass these different decisions to other brain areas.

“The final job of the CA3 region is to make the decision: Is it the same or is it different?” Knierim said. 

“Usually you are correct in remembering that this person is a slightly different version of the person you met years ago. But when you are wrong, and it embarrassingly turns out that this is a complete stranger, you want to create a memory of this new person that is absolutely distinct from the memory of your familiar friend, so you don’t make the mistake again.”

Knierim and Johns Hopkins postdoctoral fellows Heekyung Lee and Cheng Wang, along with Sachin S. Deshmukh, a former assistant research scientist in Knierim’s lab, monitored rats as they got to know an environment and as that environment changed.

The team implanted electrodes in the hippocampus of the rats. They trained the rats to run around a track, eating chocolate sprinkles. The track floor had four different textures — sandpaper, carpet padding, duct tape and a rubber mat. The rat could see, feel and smell the differences in the textures.

 Meanwhile, a black curtain surrounding the track had various objects attached to it. Over 10 days, the rats built mental maps of that environment.

Then the experimenters changed things up. 

They rotated the track counter-clockwise, while rotating the curtain clockwise, creating a perceptual mismatch in the rats’ minds. The effect was similar, Knierim said, to if you opened the door of your home and all of your pictures were hanging on different walls and your furniture had been moved.

“Would you recognize it as your home or think you are lost?” he said. “It’s a very disorienting experience and a very uncomfortable feeling.”

Even when the perceptual mismatch between the track and curtain was small, the “pattern separating” part of CA3 almost completely changed its activity patterns, creating a new memory of the altered environment. 

But the “pattern completing” part of CA3 tended to retrieve a similar activity pattern used to encode the original memory, even when the perceptual mismatch increased.

The findings, which validate models about how memory works, could help explain what goes wrong with memory in diseases like Alzheimer’s and could help to preserve people’s memories as they age.

This research was supported by the National Institutes of Health grants R01 NS039456 and R01 MH094146 and by the Johns Hopkins University Brain Sciences Institute.

Study Finds Where Our Brain Stores the Time and Place of Memories

These are actual photos taken by the lifeblogging app of study participants. They were later shown these photos while in the fMRI and asked to recall the memories associated with the pictures.

A first view of real-life memories ‘on the scale of our lives’

Newswise, August 18, 2015 — COLUMBUS, Ohio – For the first time, scientists have seen evidence of where the brain records the time and place of real-life memories.

Results showed that the similarity of the brain activation patterns when memories were recalled was an indicator of the breadth of space and time between the actual events.

Participants in the Ohio State University study wore a smartphone around their neck with an app that took random photos for a month. Later, when the participants relived memories related to those photos in an fMRI scanner, researchers found that a part of the brain’s hippocampus stores information about where and when their specific memories occurred.

In fact, the study, published this week in the Proceedings of the National Academy of Sciences, showed that the further apart the memories occurred in space and time, the farther apart the memories’ representations appeared in the hippocampus.

“What we’re picking up here is not the whole memory, but the basic gist – the where and when of the experience,” said Per Sederberg, senior author of the study and assistant professor of psychology at Ohio State.

“This could be viewed as the memory hub, where we have these general, large-scale representations of our experiences.”

Similar work has been done in rats – in fact the discovery of rat neurons that code for space won the Nobel Prize in Medicine last year. But in rats, the space they live in can be measured in feet.

There have also been studies in humans that ask them to recall lists of words or other information that they had just seen – but that recorded memories of just a few minutes that were created under experimental conditions.

This study greatly expands on both of those dimensions, by looking at real-life memories in humans.

“We found that the hippocampus represents time and space for at least a month of memories spanning up to 30 kilometers (19 miles) in space,” Sederberg said. 

“It is the first time we’ve been able to study memories on the scale of our lives.”

Sederberg led the study with Dylan Nielson, a Ph.D. graduate of Ohio State. Other co-authors were Troy Smith and Vishnu Sreekumar of Ohio State and Simon Dennis, a former Ohio State professor now at the University of Newcastle in Australia.

The study involved nine women aged 19 to 26 who wore an Android-based smartphone on a strap around their neck for one month. The phone was equipped with a custom lifeblogging app designed by Dennis. The app would take photos at random times of the day, recording the time, location, whether the person was moving and other information.

Over the course of the month, the phone took an average of about 5,400 photos for each participant.

After the month was over, the participants were placed in an fMRI scanner that measured activity in their brain while they were shown 120 of their own photos. 

Participants were asked to try to remember the event depicted in each picture and relive the experience in their mind while viewing the photo for eight seconds.

The researchers compared fMRI data on pairs of images for each participant. The photo pairs chosen were taken at least 100 meters and 16 hours apart.

Remembering an experience “lights up” many parts of the brain, but different memories create different patterns of activity. 

The more different two memories are, the more different the pattern of activity will be. Results showed that patterns of activity in the left anterior hippocampus were more different for memories of events that happened further apart in time and space.

“If the participants didn’t recall the images, we didn’t see this relationship,” Sederberg said.

“We also don’t get this effect if we only asked about the time and not the place of the memory. We found that time and space are very much intertwined in our representations of memories.”

Sederberg said the representations they found in the left anterior of the hippocampus aren’t the totality of the memories, but just the broad picture of where and when it occurred. Other research suggests that the posterior portion of the hippocampus may “fine-tune” the time and place.

“What we found may be just the targeting mechanism that gives us the general gist of the memory. And then there is a process that moves out through the rest of the hippocampus and spreads out through the cortex as we relive the entirety of the memory,” he said.

Sederberg noted that the hippocampus is one of the first areas of the brain to degrade in Alzheimer’s disease.

“People with Alzheimer’s may forget experiences and people because they are not able to effectively target their old memories. They can’t retrieve memories because they can’t get the right general cue to get to that memory,” he said.

That’s one of the issues he would like to explore in future studies. Sederberg said he hopes to repeat this study with people of different ages and with people who are showing early signs of dementia to see how their brains are representing their memories.

He also plans to collect months or even a year’s worth of data to see how we target memories over even longer periods of time and greater distances.

“We’ve got a decade or more of work ahead of us. This is just the first step,” Sederberg said.